(1) Field of the Invention
The present invention relates to a vane-type rotary compressor. More particularly, the present invention relates to an improved vane-type rotary compressor having a cylinder bore profile specifically designed to reduce fluctuation of overall drive torque usually resulting during operation of a rotary compressor. The rotary compressor according to the invention may be suitably used in an air-conditioning system of an automobile for pumping a refrigerant circulated in the system.
(2) Description of the Related Art
Typically, a vane-type rotary compressor includes a pump cylinder having a substantially cylindrical bore defining a pumping chamber. A pump rotor driven by a rotor shaft is rotatably received within the cylinder bore and is offset from the central axis of the bore in such a manner that the outer periphery of the rotor inscribes the inner wall of the cylinder bore. The rotor is provided with a plurality of angularly spaced, substantially radial vane slots in which a plurality of movable vanes are slidably fit, with their sealing edges in a close sliding contact with the inner wall of the cylinder bore. The pumping chamber is divided by the slidable vanes into a plurality of variable volume working chambers, each defined between two consecutive vanes. Due to the offset arrangement of the rotor, each vane projects from and retracts within the rotor as the rotor is rotated, so that the volumes of the respective working chambers are cyclically varied between the minimum and maximum values, thereby performing in sequence intake, compression, and delivery strokes of the compressor.
The torque required to rotationally drive any particular single vane is primarily dependent on the differential pressure developed between the leading and trailing sides of that vane due to the high pressure fluid in the preceding working chamber located at the leading side of the vane and the low pressure fluid in the successive working chamber located at the trailing side of the same vane. More specifically, the torque for a particular vane is determined by the product of the differential pressure multiplied by the surface area of the portion of the vane projecting from the rotor and by the distance of that portion measured from the central axis of the rotor.
When the vane is moving on an intake stroke, in other words, when the working chamber located at the leading side of the vane is under suction pressure, the vane undergoes substantially no differential pressure so that the drive torque required to rotate the vane is negligible. When the vane is moving on the compression stroke, the differential pressure increases but the amount of projection of the vane from the outer periphery of the rotor decreases. When the vane is moving on the delivery stroke, the differential pressure and the amount of vane projection both decrease. In an ordinary vane-type rotary compressor, the drive torque required to move an individual vane sharply increases and peaks at about the end of the compression stroke and suddenly drops thereafter. This causes the overall drive torque to fluctuate, thereby giving rise to vibrations and noise during the compressor operation. Such vibrations and noise are particularly disadvantageous when the compressor is mounted on an automobile as a pump for a refrigeration system. In one complete revolution of the rotor shaft, there are as many fluctuations in the drive torque as the number of slidable vanes and as the compression strokes performed in one revolution.
One solution to the problem of drive torque fluctuation has been proposed in Japanese Unexamined Utility Model Publication (Kokai) No. 58-106580, published July 20, 1983. In FIG. 3 of that publication, there is disclosed a rotary vane compressor having a triangular pump cylinder. The cylinder 11 is provided with three independent pumping chamber 19 each adapted to cooperate with four slidable vane 18. During one revolution of the rotor 12, each of the four vanes undergoes drive torque variations as shown in the lower part of the graph of FIG. 4 so that the resultant torque or overall torque required to drive the compressor is relatively flattened as shown by the curve A. However, this arrangement has the disadvantage that it requires a plurality of delivery ports and delivery valves, thus the increasing number of parts and members.
Japanese Unexamined Patent Publication No. 58-70086 proposes to solve the problem of torque fluctuation in a different way. This proposal is based on the principle that, in order to reduce torque fluctuation, the profile of cylinder bore must be determined in such a manner that for each vane moving on the compression stroke, the amount of vane projection decreases as a function of the pressure increase. Toward this end, as shown in the graph of FIG. 4 of that publication illustrating the amount of vane projection in terms of the angle of rotation, the cylinder profile is composed of, for each section forming one complete cycle of intake, compression, and delivery strokes, a circular sealing section Q.sub.0 -Q.sub.1 in which the rotor contacts the cylinder inner wall, a curved region Q.sub.1 -Q.sub.2 in which the vane is projected with an increasing projection speed, a curved region Q.sub.2 -Q.sub.3 in which the projection speed of the vane is decreased, a circular region Q.sub.3 -Q.sub.4 in which the amount of projection is kept constant, a curved region Q.sub.4 -Q.sub.5 in which the vane is retracted with an increasing retraction speed, a curved region Q.sub.5 -Q.sub.6 in which the retraction speed is decreased, a circular region Q.sub.6 -Q.sub.7 in which the amount of vane projection is held constant, a curved region Q.sub.7 -Q.sub.8 in which the vane is further retracted with an increasing retraction speed, and a curved region Q.sub.8 -Q.sub.9 in which the retraction speed is decreased. The disadvantage of this solution is that the followability of the vane (i.e., the ability of the vane to follow the inner circumference of the cylinder bore) becomes poor because the vane must be projected with a considerable rate of acceleration.